Worldwide-scale introduction of a ‘Code Division Multiple Access (CDMA)’ method has recently been pursued in the field of mobile communications being used compact mobile communications device such as cellular phones, personal-handy phones or microcellular phones. In association with such a tendency, an intermodulation distortion (hereinafter called ‘IMD’) of a non-reciprocal device used in mobile communications, such as an isolator or a circulator, has come to gain attention despite the IMD having thus far posed no problem in a conventional analog communications scheme. The IMO corresponds to an undesired signal which would additionally arise when two or more signals are supplied to a non-linear device, which is used in non-reciprocal device. For instance, when two signals, one having a frequency f1 and the other having a frequency f2, are simultaneously input to a non-reciprocal device, there arise frequency components other than the signals of frequencies f1 and f2 such as a sideband having a frequency of (2f1−f1) and a sideband having a frequency of (2f2−f1). These sidebands will cause crosstalk or noise. Therefore, sidebands must be suppressed.
The IMD can be suppressed, by applying a sufficiently strong direct current magnetic field (hereinafter called ‘d.c. magnetic field’ simply) to a ferrimagnetic member (i.e. a member made of ferrimagnetic material) from a magnet provided in a non-reciprocal device. However, application of the d.c. magnetic field involves production of side effects; that is, shifting of a frequency band assigned to a non-reciprocal device toward a higher frequency and narrowing of the frequency band of interest, thus degrading the performance of the non-reciprocal device. Further, demand for a more compact and thinner non-reciprocal device hinders application of a sufficient d.c. magnetic field to the ferrimagnetic member.
The compact mobile communications device is battery-powered, and then, use of a low-loss device is indispensable for realizing long-time operation of the compact mobile communications device. In a case where the compact mobile communications device is equipped with a non-reciprocal device, the compact mobile communications device is also desired to have a low-loss characteristic. Not only a compact mobile communications device serving as a ‘Terminal Station’ but also an apparatus serving as ‘Base Station’ provides small coverage areas, and hence an amplifier for use of a small amount of power has been used for the apparatus. Further, a non-reciprocal device to be used as a Base Station is also desired to have a low-loss characteristic.
Also, important characteristics of a ferrimagnetic material used in a non-reciprocal device are to have a characteristic of a sufficiently low ferromagnetic resonance linewidth (hereinafter called ‘FMR linewidth’ and also represented as ‘ΔH’), which acts as a magnetic loss term.
In a non-reciprocal device, the ferrimagnetic member is used in combination with a magnet, and hence a saturation magnetic flux density of the ferrimagnetic material forming the ferrimagnetic member is ideal in assuming that a temperature coefficient may compensate for the temperature characteristic of a magnet. A close correspondence exists between the temperature coefficient of the saturation magnetic flux density and curie temperature (hereinafter represented as ‘Tc’). Ferrimagnetic material is usually desired to have high curie temperature in response to a magnet which is less susceptible to temperature change.
Japanese Patent Publication No. 31288/1981 (Kokoku 56-31288) describes the technique of arbitrarily changing the value of saturation magnetic flux density, by changing the composition ratio of yttrium-calcium-vanadium-iron garnet ferrite (hereinafter called or represented as ‘Y—CaV—Fe garnet ferrite’) substituted by indium (In) and aluminum (Al).
However, the above-mentioned material, i.e. the Y—CaV—Fe garnet ferrite, has a curie temperature as low as 160° C. or less. Therefore, a non-reciprocal device formed of the Y—CaV—Fe garnet ferrite encounters a practical problem of the non-reciprocal device being limited to use at a certain temperature or below. Further, In is a rare natural resource, and hence a ferrite containing In would become costly.